In fact, sales of its predecessor, Windows Mobile 6.5, topped those of Windows Phone 7 during the final three months of 2010, NPD said. But Windows Phone 7 wasn’t released in the U.S. until Nov. 8.

“With its mid-quarter launch Windows Phone 7 entered the epicenter of competition between (Apple’s) iOS and (Google’s) Android at AT&T. Both competitors offer mature feature sets and large app libraries,” Ross Rubin, NPD’s executive director of industry analysis, said in a news release.

“Microsoft has made the case for Windows Phone 7’s differentiation and improved integration. Now, the company must close the feature gap, offer more exclusive capabilities, work with partners to deliver hardware with better differentiation, and leverage its extensive experience in driving developer communities to increase its app offerings.”

NPD said Windows Phone 7 debuted with a lower share than Android and Palm’s webOS did when those platforms first launched. Android is now the top-selling platform in the U.S., according to NPD — and in the entire world, according to another research firm, Canalys.

Though we still don’t have concrete sales numbers for Windows Phone 7, the picture is becoming clearer: Microsoft has a mammoth task ahead of it. The Redmond-based company has remained fairly realistic in its expectations, knowing it is pushing an entirely overhauled platform into a highly competitive environment dominated by Google and Apple.

Last week, Microsoft said it sold more than 2 million Windows Phone 7 licenses to phone manufacturers by the end of December. But that number doesn’t reflect the actual number of Windows Phones sold to consumers — a figure Microsoft has declined to reveal.

Microsoft must remain aggressive during the next year if it wants to make real headway with Windows Phone 7. Especially now that the Apple iPhone is coming to Verizon, before Microsoft has a chance to get any Windows Phones on the nation’s most popular wireless network.